fix: TTS hot-swap actually works + GUI max-width + Python builder discoverable

ROOT CAUSE ANALYSIS

User reported: (a) TTS dropdown doesn't apply, (b) GUI feels stretched,
(c) no link to command builder, (d) Python doesn't even start.

Issues (a), (b), (c) addressed here. (d) addressed in python repo.

=== Fix A: TTS hot-swap (was completely broken) ===

Two distinct bugs:
1. `tts::init_backend()` only read JARVIS_TTS env var, NEVER consulted
   `Settings.tts_backend` in the DB. So saving "Silero" in the GUI
   dropdown changed nothing — not even on restart.
2. `BACKEND: OnceCell<...>` couldn't be replaced at runtime, so even
   if init had picked the right backend, the dropdown was useless
   mid-session.

Fixed:
- `tts::BACKEND` is now `Lazy<RwLock<Option<Arc<dyn TtsBackend>>>>`.
- `choose_backend_name()` resolves from DB → env → auto-detect.
- New `swap_to(name)` replaces the live backend atomically.
- IPC: new `IpcAction::SwitchTts { backend }`. jarvis-app reacts to
  it the same way it reacts to SwitchLlm.
- GUI's `set_tts_backend` now persists + hot-swaps GUI's TTS + Svelte
  page also fires `switchDaemonTts()` over WS so the running daemon
  installs the new backend.
- Removed "Применится при следующем запуске" hint — now applies
  immediately. RU + EN locale strings updated.

=== Fix B: GUI max-width ===

main + #header had no max-width, so on a 1920px monitor the labels
sprawled across 1860px of space, which felt stretched. Added
max-width: 1280px + margin: 0 auto + box-sizing: border-box. Smaller
windows are unaffected.

Also added the same cap on .app-container.assist-page so the home
screen arc reactor centres properly.

=== Fix C: Command builder discoverable ===

The Python fork ships a separate yaml-editing tool at
`python/tools/command_builder/` (pywebview GUI). No way to find it
from the main Rust GUI. Fixed:

- New tauri command `open_command_builder` (crates/jarvis-gui/src/
  tauri_commands/builder.rs). Locates the Python fork via
  $JARVIS_PYTHON_DIR → sibling python/ → C:\Jarvis\python, prefers
  the project's .venv, spawns `python -m tools.command_builder`.
- New blue "Конструктор команд (Python)" button on /settings page
  near the Save / Back buttons.

Tests: 140 rust + 104 python all pass. Release builds of jarvis-app
and jarvis-gui both green.

crates/jarvis-core/src/i18n/locales/en.ftl

@@ -153,7 +153,7 @@ settings-ai-tips = Tips
 settings-llm-backend = LLM backend
 settings-llm-backend-desc = Where to run LLM requests. Hot-swap, no restart needed.
 settings-tts-backend = TTS backend
-settings-tts-backend-desc = Takes effect on next jarvis-app launch.
+settings-tts-backend-desc = Applied immediately — no restart needed.
 settings-llm-context = Conversation context
 settings-llm-context-desc = LLM remembers previous turns. Reset if answers get weird.
 settings-llm-reset = Reset context

crates/jarvis-core/src/i18n/locales/ru.ftl

@@ -153,7 +153,7 @@ settings-ai-tips = Подсказки
 settings-llm-backend = LLM движок
 settings-llm-backend-desc = Где исполнять LLM-запросы. Hot-swap, без рестарта.
 settings-tts-backend = TTS движок
-settings-tts-backend-desc = Применится при следующем запуске jarvis-app.
+settings-tts-backend-desc = Применяется сразу — без перезапуска.
 settings-llm-context = Контекст разговора
 settings-llm-context-desc = LLM помнит предыдущие реплики. Сбросить, если ответы становятся странными.
 settings-llm-reset = Сбросить контекст

crates/jarvis-core/src/ipc/events.rs

@@ -76,6 +76,10 @@ pub enum IpcAction {
     // Reload LLM backend from settings DB (after GUI changed it independently).
     ReloadLlm,
 
+    // Daemon-side TTS hot-swap. GUI fires this after persisting to DB so the
+    // running daemon installs the new backend without a restart.
+    SwitchTts { backend: String },
+
     // Request a health snapshot — daemon responds via IpcEvent::HealthSnapshot.
     QueryHealth,
 }

crates/jarvis-core/src/tts/mod.rs

@@ -12,7 +12,6 @@
 //!
 //! If `JARVIS_TTS` is unset, the dispatcher auto-detects Piper, otherwise SAPI.
 
-use once_cell::sync::OnceCell;
 use std::path::Path;
 use std::sync::Arc;
 
@@ -84,11 +83,41 @@ pub trait TtsBackend: Send + Sync {
     fn speak(&self, text: &str, opts: &SpeakOpts);
 }
 
-static BACKEND: OnceCell<Arc<dyn TtsBackend>> = OnceCell::new();
+// Live backend swap: stored behind an RwLock<Arc<...>> so reads are cheap
+// (clone the Arc) and writes can replace the whole backend instance
+// without invalidating outstanding speaks. The OnceCell pattern we used
+// before couldn't be replaced at runtime — the GUI's TTS dropdown ended up
+// only persisting the choice without applying it. Now `swap_to(name)`
+// installs a fresh backend immediately.
+static BACKEND: once_cell::sync::Lazy<parking_lot::RwLock<Option<Arc<dyn TtsBackend>>>> =
+    once_cell::sync::Lazy::new(|| parking_lot::RwLock::new(None));
 
-/// Get the active TTS backend. Initializes once on first call.
+/// Get the active TTS backend. Initialises lazily on first call. The choice
+/// comes from (in order): persisted Settings DB → `JARVIS_TTS` env var → auto.
 pub fn backend() -> Arc<dyn TtsBackend> {
-    BACKEND.get_or_init(init_backend).clone()
+    if let Some(b) = BACKEND.read().clone() {
+        return b;
+    }
+    let chosen = choose_backend_name();
+    let inst = build_backend(&chosen);
+    *BACKEND.write() = Some(inst.clone());
+    inst
+}
+
+/// Swap to a named backend at runtime — used by the GUI's TTS dropdown.
+/// `name` is "sapi" / "piper" / "silero" / "" or "auto" (re-runs auto-detect).
+/// Returns the resolved backend name on success, error string otherwise.
+pub fn swap_to(name: &str) -> Result<&'static str, String> {
+    let normalised = name.trim().to_lowercase();
+    let target = match normalised.as_str() {
+        "" | "auto" => choose_backend_name(),
+        "sapi" | "piper" | "silero" => normalised,
+        other => return Err(format!("unknown TTS backend '{}'", other)),
+    };
+    let inst = build_backend(&target);
+    let resolved = inst.name();
+    *BACKEND.write() = Some(inst);
+    Ok(resolved)
 }
 
 /// Speak a text via the active backend. Applies sanitisation unless `opts.raw` is set.
@@ -109,19 +138,35 @@ pub fn speak_default(text: &str) {
     speak(text, &SpeakOpts::default());
 }
 
-fn init_backend() -> Arc<dyn TtsBackend> {
-    let choice = std::env::var("JARVIS_TTS").ok()
+/// Resolve the backend name from (DB → env → auto). Pure logic, no instantiation.
+fn choose_backend_name() -> String {
+    // 1. Persisted Settings DB. Empty string means "auto".
+    if let Some(db) = crate::DB.get() {
+        let from_db = db.read().tts_backend.trim().to_lowercase();
+        if !from_db.is_empty() {
+            return from_db;
+        }
+    }
+    // 2. JARVIS_TTS env var.
+    let from_env = std::env::var("JARVIS_TTS").ok()
         .map(|s| s.trim().to_lowercase())
         .unwrap_or_default();
+    if !from_env.is_empty() {
+        return from_env;
+    }
+    // 3. Auto-detect: prefer Piper if installed, else SAPI.
+    if PiperBackend::try_new().is_ok() { "piper".into() } else { "sapi".into() }
+}
 
-    match choice.as_str() {
+fn build_backend(choice: &str) -> Arc<dyn TtsBackend> {
+    match choice {
         "piper" => match PiperBackend::try_new() {
             Ok(b) => {
                 info!("TTS backend: Piper ({} / {})", b.binary_display(), b.voice_display());
                 return Arc::new(b);
             }
             Err(e) => {
-                warn!("JARVIS_TTS=piper requested but unavailable ({}). Falling back to SAPI.", e);
+                warn!("Piper requested but unavailable ({}). Falling back to SAPI.", e);
             }
         },
         "silero" => match SileroBackend::try_new() {
@@ -130,21 +175,11 @@ fn init_backend() -> Arc<dyn TtsBackend> {
                 return Arc::new(b);
             }
             Err(e) => {
-                warn!("JARVIS_TTS=silero requested but unavailable ({}). Falling back to SAPI.", e);
+                warn!("Silero requested but unavailable ({}). Falling back to SAPI.", e);
             }
         },
-        "sapi" | "" => {
-            // Auto-detect: prefer Piper if installed, else SAPI.
-            if choice.is_empty() {
-                if let Ok(b) = PiperBackend::try_new() {
-                    info!("TTS backend: Piper (auto-detected, {} / {})", b.binary_display(), b.voice_display());
-                    return Arc::new(b);
-                }
-            }
-        }
-        other => {
-            warn!("Unknown JARVIS_TTS value '{}' — using SAPI.", other);
-        }
+        "sapi" => {}
+        other => warn!("Unknown TTS choice '{}' — using SAPI.", other),
     }
 
     info!("TTS backend: SAPI (Windows built-in).");